The present invention relates to a mixture comprising microporous, oxidic inorganic material with or without opacifier, fiber material and inorganic binding material, which is compression-molded to a heat-insulating molding.
Heat insulating moldings are known and described, for example, in U.S. Pat. No. 4,985,163. They comprise microporous, oxidic inorganic material, for example, pyrogenic silicas, including arc silicas, precipitated silicas or silicon dioxide aerogels and aluminas prepared analogously. To achieve good heat insulation properties the material has high specific surface areas which are in the range of 50-700 m.sup.2 /g. Because of the large surface areas, the adsorption capacity for polar substances is highly pronounced so that in a natural atmosphere the material eagerly takes up water. Similar properties are exhibited by the heat-insulating moldings produced which absorbs moisture in storage and use. If a molding of this kind is subjected within a short period to high thermal energy, there is an explosive production of water vapor, which destroys the structure of the molding. This effect occurs, for example, with heat-insulating moldings used as thermal insulators in radiant heaters for ceramic hobs, which heaters are typically brought to incandescence in from 1 to 5 seconds.
There is a need to provide for an increase in the gas permeability of such moldings, especially for an increase in the diffusion of water vapor from the interior to the surface of the molding and to avoid the occurrence of excess pressure in the interior of the molding, which would destroy its structure.
In accordance with EP 618 399 A1, a tool is used to drive channel pores mechanically into the surface of the molding. However, this method causes problems, especially when the surface of the molding is not smooth but structured and possesses, grooves, rifts or other bulges. It is difficult to avoid instances of breakoff or flakeoff of material when the channel pores are being made, in the base of the grooves or at the edge of the grooves, thereby rendering the molding unusable for the intended application.